U.S. Pat. No. 6,247,259 to Tsadka, et al., assigned to the present assignee, describes a method for the fire control of flat trajectory weapons, which comprises the steps of measuring the target range and cross wind velocity along the intended projectile trajectory prior to firing the weapon and, using the know ballistic equations of the projectile, determining the expected vertical and horizontal deflection of the projectile and adjusting the weapon sight to compensate for said deflections.
More specifically, the method of U.S. Pat. No. 6,247,259 comprises, prior to firing the weapon, the steps of generating a laser beam at the firing position, receiving the beam reflected by the desired target, determining the target range by measuring the time lag between the generation of said laser beam and the reception of said reflected beam (viz. the double pass time of flight of the laser pulse between transmitter and target), determining the crosswind direction and velocity along the trajectory by receiving said reflected laser beam in two separate positions and measuring the intensity fluctuations of said beam in said two separate positions, determining, using the ballistic equations of the projectile, the expected vertical and horizontal deflection of the projectile, and adjusting the weapon sight to compensate for said deflections, either by a) providing the shooter with sufficient information to adjust the sight of the weapon as required by said deflections, or b) automatically adjusting said sight.
Various methods have been suggested for remote sensing of the path-averaged crosswind. Most of them depend on temporal analysis of the reflected beam intensity fluctuations that are produced when refractive-index irregularities are drifted across the beam.
By measuring a time-lagged covariance function with spaced detectors, one can find the strength and direction of the crosswind (See R. S. Lawrence et al, in Applied Optics, Vol.11 (1972), No.2, pp. 239-243). The limitations of systems based on this technique become apparent when paths longer than 500 m are probed. For cases of strong refractive turbulence strength Cn2, scintillations saturate, and the system performance becomes unpredictable. Moreover, such a system is sensitive to non-uniformities along the propagation path (e.g., turbulence strength changes, wind fluctuations).
Ting-I Wang et al, in Applied Optics, Vol.20 (1981), No.23, pp. 4073-4081, compared various methods with regard to their immunity to the saturation problem as well as to Cn2 and wind fluctuations along the propagation path. Their conclusion was that a frequency technique (FT), involving counting zero-crossings of the mean of the signal or width of the autocorrelation function analysis, is superior to other techniques. Nevertheless, no technique is ideal and the FT technique has its own limitations, mainly due to turbulence spectrum changes.
L. C. Andrews et al, in J. Opt. Soc. Am, Vol.16 (1999), No.6, pp. 1417-1429 (herein “L. C. Andrews et al.”), in their heuristic model of optical scintillation, showed the existence of a definite form of coupling between the turbulence strength and the turbulence spectrum. This model was developed under the assumption that the turbulence spectrum is characterized by a two-scale behavior, one (small-scale) for diffractive irradiance fluctuations and another one (large scale) for refractive irradiance fluctuations. In accordance with this model it is possible to define the upper frequency bound (for the smallest cell size) and the lower frequency bound (for the largest cell size) in strong and in weak turbulences. Thereby it is possible to apply an envelope filter for the FT technique.